Early
Digital Test
Equipment
Fluke 8100B
DVM
 

Manufacturer: Fluke
Model: 8100B
Year: 1973 (1969)
Schematic: MF
 
Functions: DCV, ACV, Ohms
Display: NIXIE
Digits: 4
Logic: DTL,TTL

This model is based on another novel A-D conversion technique called "Recirculating-Remainder A-D Conversion" by Fluke.

The technique can be described as performing long division with analog values. A counter, resistive D-A ladder and comparator are arranged in a typical staircase A-D loop but this arrangement is only 4 bits wide: only enough for one decade of resolution. The process starts by using this single-digit A-D converter to resolve the most significant digit. The analog difference between the input value and the D-A ladder (the remainder) is stored in a sample-and-hold circuit, multiplied by 10 through an op amp, and fed to the single-digit A-D converter to resolve the second digit. The process is then repeated to resolve the 3rd and 4th digits.

This results in a minimal component count on the digital side with only 3 digital ICs: a 4-bit counter, a NIXIE decoder/driver and a triple NAND gate. On the analog side numerous JFETs are used as analog switches.

This model has auto-polarity but not auto-ranging.

 



- Unit Log -

Serial Number: 58412
Year of Manufacture: 1973 (transistor date code of 7301).
Date of Receipt: 11 May 2008
State upon Receipt: Good physical condition but not functional. Display stuck at 2222 on all functions.


Date: 14 May 2008
Symptoms: Display stuck at all 2s.
Analysis: All 3 digital ICs found to be faulty.
Solution: Replaced. 839 DTL 4-bit counter replaced with 74LS197. Display now varies in accordance with applied voltage, but is way off linear, making large jumps along the way.


Date: 14 May 2008
Procedure: Trim pots marked to indicate original positions, then turned to clear any bad contact problems.


Date: 18 May 2008
Symptoms: Continuing from 14 May.
Analysis: Stepping function during count as seen at TP2 doesn't seem quite right. Surmise that there is a problem around the primary DA ladder. Output from Q43,Q44 not changing. BE junction of Q45 is open.
Solution: Q45 replaced. Unit is now functional in DCV and KOhm modes, and fairly accurate even with no recalibration. ACV mode still has a problem.


Date: 18 May 2008
Procedure: Cleaned.


Date: 24 May 2008
Symptoms: ACV mode does not work properly (cont'd from 18 May 2008).
Analysis: TP9 shows a negative voltage during input sample phase (1) with in-range AC input, whereas it should be positive. Suspect a problem with Q16 as the only negative voltage nearby is the gate of Q16 during other phases.
Solution: Q18 moved to Q16. 2N5459 inserted for Q18. (Vdss of 2N5459 is 25V rather than preferred 30V.) Unit reassembled. Fully functional.



  .. | Fluke 8100B | ..
Early Digital Test Equipment
bhilpert